Currently, several injection systems are commercially available for subcutaneous substance delivery of medication. Pen-type injection systems typically use 29 to 31 gauge needles having lengths of between about 5 mm and 12.7 mm, and are used to deliver the contents of a medication cartridge, such as insulin, to the subcutaneous tissue layers of a patient rapidly and conveniently. Additional details of intradermal drug delivery have been previously described in U.S. patent application Ser. No. 09/835,243, filed Apr. 13, 1999, and Ser. No. 09/417,671 filed Oct. 14, 1999, the entire content of each application being incorporated herein by reference.
A “microneedle” pen system has also been developed that further serves to reduce the pain and sensation to the user normally experienced with subcutaneous substance delivery. Such microneedle drug delivery systems may include shorter needles, typically less than or equal to about 3 mm, and having smaller diameters, typically in the range of between about 30 to 34 gauge or thinner. Such needle length and gauge design combinations are desirable to provide for sharp, yet short, point geometries that can more accurately target substance delivery, thereby permitting controlled delivery to only certain selected tissue, such as the deep intradermal or shallow subcutaneous tissue layers. Current typical pen injection systems used for subcutaneous delivery are not believed to be optimal for use by the general population of self-injectors because of, inter alia, the high backpressures associated with injecting fluid into the intradermal layers of the skin using the microneedles.
To achieve effective delivery in light of higher backpressure, it is desirable to control two factors: the depth accuracy of the injection and the rate of the injection. The delivery of medicament within the narrow depth range of the intradermal tissue layer should first be assured and maintained during injection. Once the depth accuracy is obtained, the rate of injection should be controlled to minimize or eliminate leakage of the medicament. Additional details of intradermal drug delivery and microneedles have been previously described in U.S. Pat. No. 6,494,865, issued on Dec. 17, 2002, U.S. Pat. No. 6,569,143, issued on May 27, 2003, and related U.S. patent application Ser. No. 10/238,958, filed Sep. 11, 2002, all of which are assigned to Becton, Dickinson and Company, and the entire contents of each such patent and application being incorporated herein by reference.
The intradermal tissue layer of the skin is considerably denser than the subcutaneous tissue region. The density of the intradermal tissue space on a particular patient is, in part, a function of the collagen make-up which is affected by the patient's age and the location of the injection site on the patient's body. This increased density of the intradermal tissue layer can create greater backpressure resistance on the injection device than the resistance created when injecting into the subcutaneous tissue region. To overcome the increased backpressure resistance when injecting into the intradermal tissue layer with a conventional pen system, the user or patient must exert greater force or pressure (which could be substantial), on the injector device actuator or employ an injector device incorporating a means for generating a mechanical advantage. In these applications, the injector device should preferably be designed to withstand the greater backpressure from the intradermal injection site, as well as facilitate the use of additional force or pressure exerted by the user or patient. It must be noted that the increased fluid pressure of the medicament required to actuate the injector device, if not carefully controlled, may result in “jetting” the medicament past the desired tissue depth, which would be undesirable.
Conventional pen-type injection systems may also require that the user keep the needle seated in the skin for a period of up to 10 seconds to allow the “axial compliance” of the pen mechanism (or a lead screw) and the cartridge back-end stopper to equilibrate to minimize “drool” from the needle tip upon withdrawal.
Therefore, a need exists to provide an injection pen device and method that incorporates a feature to prevent or minimize leakage of a medicament during and after delivery by ensuring accurate injection depths and optimum injection rates.